The broad long-term objective of this proposal is to understand the pathogenesis of NK leukemia. The mechanisms responsible for expansion of leukemic NK cells are not known. The central hypothesis of this proposal is that NK leukemia results from dysregulated apoptosis. Leukemic NK cells are relatively resistant to Fas-mediated death despite expressing high levels of both Fas receptor and Fas ligand. Reversal of Fas resistance after IL-2 activation suggests inhibition of Fas signaling in leukemic NK cells. Experiments in specific aim 1 are directed at determining mechanisms of Fas-resistance in leukemic NK cells. Initial experiments will focus on the role of novel, inhibitory decoy Fas receptors, which we have cloned from leukemic NK cells. The hypothesis that MAPK signaling contributes to Fas resistance will also be examined in specific aim 1. Leukemic NK cells show constitutive activation of MAPK; furthermore inhibition of MEK/MAPK reverses Fas resistance. Pharmacological inhibition of MEK/MAPK and dominant negative (DN) MEK proteins expressed in vaccinia virus will be utilized to examine mechanisms of MAPK downstream signaling leading to Fas resistance. We hypothesize that leukemic NK cells are activated cytotoxic cells dependent on survival signals resulting from target recognition in vivo. Preliminary data show that ligation of an unknown NK activating receptor induces MAPK activation and modulates Fas sensitivity in an NK cell line. Experiments in specific aim 2 will examine mechanisms of NK receptor signaling upstream of MAPK activation. Specific NK receptors responsible for protecting leukemic NK cells from Fas-induced death will be identified. Experiments in specific aim 3 will examine the hypothesis that constitutive STAT signaling regulates apoptotic resistance. Preliminary data show constitutive STAT activation in leukemic NK cells; furthermore, inhibition of STAT activation directly induces apoptosis in leukemic NK cells as well as conferring Fas-sensitivity. The experimental strategy will utilize specific inhibitors of tyrosine kinases that regulate STAT activation as well as DN- STAT proteins. These studies are important for understanding the pathogenesis of NK leukemia. Results of these studies should identify molecular targets in MAPK and STAT signaling pathways important for therapeutic development in hematologic malignancies.